Method and apparatus for linearizing an output signal

ABSTRACT

The digital transmission sub-system comprises a DSP ( 100 ) which takes as its inputs the in-phase and quadrature channels of a baseband software radio stage. The in-phase and quadrature inputs are separately digitally predistorted ( 110, 120 ) and digitally up-converted ( 128 ) to the intermediate frequency (IF) band. The IF band signal is then converted ( 130 ) to an analogue signal which is up-converted to the radio frequency band at ( 134 ) prior to amplification at ( 140 ) to produce an RF output signal for radiation from an antenna. The predistors ( 110  and  120 ) predistort the incoming signals to counter distortion arising from the up-conversion and amplification processes within the transmission subsystem in order to linearise the RF output. The predistorters ( 110, 120 ) may be adapted using a feedback signal supplied to the DSP ( 100 ) from a splitter ( 142 ) at the output of the sub-system. The feedback mechanism may involve analogue correlation processes to permit the use of slower analogue to digital converters for providing the feedback to the DSP ( 100 ) (FIGS.  4  and  5 ).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of International Application No.PCT/GB00/01220 filed Mar. 30, 2000 and published in English, which inturn claims priority based on GB 9907725.7 filed Apr. 1, 1999.

FIELD OF THE INVENTION

This invention relates to signal processing apparatus of the kind inwhich an input signal is subject to both amplification and frequencyconversion; and especially relates to radio telecommunications apparatusin which a voice signal is subject to amplification and frequencyconversion.

BACKGROUND OF THE INVENTION

The emerging GSM-EDGE and UMTS standards for mobile telecommunicationsplace an increasingly stringent requirement on the linearity ofhandsets, particularly given their proposed wider channel bandwidths. Inorder to realise a power-efficient handset design, some form oflinearisation will be required in the handset transmitter which shouldbe (i) low-power itself; (ii) capable of broadband linearisation (up to5 MHz for UMTS/ULTRA; (iii) frequency flexible, and preferablymulti-band; and (iv) capable of achieving and maintaining high-levels oflinearity improvement with highly-non-linear power amplifiers (e.g.class-C).

According to one aspect, the invention consists in a method oflinearising an output signal comprising the steps of providing an inputsignal, digitally predistorting the input signal using polynomialdistortion generation and frequency converting it in succession toprovide a predistorted, frequency-converted signal, and amplifying thepredistorted, frequency-converted signal to produce an output signal.

The trend in base-station technology is toward the adoption of “softwareradio” techniques, i.e. architectures in which all of the modulationparameters, ramping, framing etc. take place for all channels atbaseband (digitally). The combination of all channels, at appropriatefrequency offsets from each other, can also be performed at baseband andthe whole spectrum up-converted in a single block for multi-carrierpower amplification and the transmission from a single antenna.

SUMMARY OF THE INVENTION

However, the up-conversion and power amplification need to be linear(low-distortion) in order to prevent the radiation of unwanted adjacentchannel energy and hence some form of linearisation is usually requiredfor the power amplifier. In one embodiment of the present invention, thesystem incorporates a digital baseband (or digital IF) interface betweena baseband signal generation sub-system and a linearised transmittersub-system performing the above method.

With the invention it is possible to allow the transmitter to become adigital-in, RF-out system with the linearisation taking place in theform of digital predistortion.

In one embodiment, the predistortion of the input signal occurs prior toits frequency conversion. Advantageously, the frequency converting stepis a frequency up-conversion step.

The input signal may be provided in quadrature form comprising in-phaseand quadrature channels and the predistorting step may involvepredistorting each channel independently.

Advantageously, the predistortion process may involve controlling theamplitude and/or phase of some part of the predistortion. This mayinvolve controlling the predistortion to introduce a variation ofamplitude and/or phase with frequency into at least a part of thepredistortion.

In one embodiment, the predistortion may be controlled on the basis of afeedback signal derived from the output signal. In such an embodiment,it is possible to inject a pilot signal into the input signal and tomonitor distortion of the pilot signal in the output signal as feedback.The feedback may be used together with the generated predistortion togenerate control signals controlling the predistortion. The generationof these control signals may involve the step of correlating, or mixing,predistortion with feedback. It may be advantageous to perform the stepof using the predistortion together with the feedback signal to generatecontrol signals for the predistortion at least partly in the analoguesignal domain.

The predistorting process may involve generating a distortion from theinput signal and reintroducing the generated distortion into the inputsignal. The distortion signal may be generated by mixing or multiplyingthe input signal with itself. The step of generating a predistortion mayinvolve generation of different orders of distortion by mixing the inputsignal with itself a different number of times. Advantageously,different orders of distortion can be controlled separately.

In a preferred embodiment, the frequency conversion and predistortionprocesses occur within a digital signal processor.

Any of the various methods described above may be used to generate anoutput signal for transmission from antenna means using an input signalwhich has been created in the digital domain and which containsinformation which it is desired to transmit.

According to another aspect, the invention relates to apparatus forlinearising an output signal comprising predistorting means fordigitally predistorting the input signal using polynomial distortiongeneration and frequency converting means operating in succession on aninput signal to produce a predistorted, frequency-converted signal, theapparatus further comprising amplifying means for amplifying thepredistorted, frequency-converted signal to produce an output signal.

Certain embodiments of the invention will now be described, by way ofexample only, with reference to the figures, in which:

FIG. 1 is a diagram illustrating a digital transmission sub-systemlinearisation scheme;

FIG. 2 is a diagram of a predistorter;

FIG. 3 is a diagram of a non-linearity generating circuit;

FIG. 4 is a diagram illustrating a digital transmission sub-systemlinearisation scheme;

FIGS. 5(a-c) is a diagram illustrating a digital transmission sub-systemlinearisation scheme; and

FIG. 6 is a diagram illustrating a predistortion circuit.

FIG. 1 illustrates a basic digital transmitter linearisation systemutilising polynomial-based predistorters. The baseband, digital inputsignal to the system is provided by, for example, a “software radio”architecture in which all of the modulation parameters, ramping,framing, etc. take place for all channels digitally at baseband. Thisinput for the transmitter system of FIG. 1 is provided in the form ofdigital in-phase and quardrature channel inputs, I and Q respectively,which are supplied to digital signal processor (DSP) 100.

The in-phase channel input signal is digitally predistorted usingin-phase channel polynomial predistorter 110, whereas the quadraturechannel input signal is digitally predistorted using quadrature channelpolynomial predistorter 120. The outputs from predistorters 110 and 120are mixed, using mixers 122 and 124 respectively, into in-phase andquadrature versions respectively of a signal from local oscillator 126by way of quadrature splitter 128. The outputs from mixers 122 and 124are then combined digitally to produce an intermediate frequency (IF)band output signal which is converted to an analogue signal by digitalto analogue converter 130. The analogue IF output signal is thenbandpass filtered at 132 and, using mixer 134, is subsequently mixedwith the output from local oscillator 136 to produce a signalup-converted to the radio frequency (RF) band. This RF signal is thenbandpass filtered at 138 prior to being amplified by non-linear RF poweramplifier 140 which provides the system output to for example, anantenna of a hand-set or base station. The purpose of the predistorters110 and 120 in the DSP 100 is to compensate for the non-linearcharacteristics of the RF power amplifier (PA) 140, and possibly also ofthe up-conversion process, in order to linearise the response of theentire transmitter system.

The predistorters 110 and 120 function by applying a predistortion tothe I and Q input channels respectively which compensates for thedistortion caused by the PA 140 (and possibly also by the up-conversionprocess). The characteristics of the predistortions applied at 110 and120 are controlled on the basis of a feedback signal derived from theoutput of PA 140 using splitter 142. The portion of the PA output fedback from this splitter is coherently downconverted by mixing it withthe output of local oscillator 136 which is used to up-convert the IFsignal in the main signal path. The result of this mixing process, whichtakes place at mixer 144, is filtered at 146 prior to being converted toa digital signal at 148 which is supplied to the DSP 100 in order toprovide feedback control for the predistorters 110 and 120.

The transmitter system of FIG. 1 can be adapted in a number of ways. Forexample, the DSP 100 could be provided with analogue to digitalconverters at the in-phase and quadrature channel inputs in order toprovide compatibility with an analogue baseband stage, rather than a“software radio” architecture as discussed above. Furthermore, the inputto the DSP 100 could be a digital or analogue IF band input signal,which could be quadrature downconverted digitally (after any necessaryanalogue to digital conversion) in the DSP prior to being processed asdiscussed above with reference to FIG. 1. Further modifications couldalso be made. For example, multi-stage upconversion from the IF to theRF band could be employed and/or an amplitude and phase polynomial modelcould be used in place of the in-phase and quadrature (Cartesian) modelemployed in FIG. 1. The up-conversion of the predistorted signal to theIF band and beyond can take place in the analogue signal domain.

FIG. 2 illustrates the form of the predistortion circuit employed foreach of the predistorters 110 and 120 used in the FIG. 1 system. Thein-phase or, as the case may be, quadrature channel input signal isprovided to splitter 200 which distributes it to the various componentsof the predistorter to generate various orders of distortion (to beexplained later). For example, the input signal from splitter 200 isprovided to third order non-linearity generator 210 to generate a thirdorder non-linearity which is gain and phase adjusted at 212 and 214respectively, before being supplied to combiner 216. Any additionalorders of distortion, for example, the fifth, seventh and nth, aregenerated and adjusted in a similar manner, and are supplied to combiner216.

In the combiner 216, the adjusted non-linearities are recombined withthe input signal to the splitter 200 which passes to the combiner 216 byway of delay element 218 which compensates the input signal for thedelay experienced by the signals in the non-linearity generating pathsfor the various orders of distortion. Thus, the signal output fromcombiner 216 to mixer 122 or, as the case may be, mixer 124 comprisesthe sum of the predistorter input signal and the independently adjustedand generated orders of distortion.

A process by which the different orders of distortion can be generatedfor independent control will now be described with reference to FIG. 3.Essentially, each order of distortion is created by multiplying an inputsignal (i.e. the in-phase or quadrature digital input channel assupplied to the splitters 200 in predistorters 110 and 120) with itself.This process is described in detail in UK Patent Application 9804745.9.In FIG. 3, the input signal, in addition to being supplied to delayelement 218 in FIG. 2, is supplied to splitter 300 which thus performsthe function of splitter 200 in FIG. 2.

In the FIG. 1 embodiment, a high speed analogue to digital converter maybe required to sample the IF band feedback signal. The embodiment ofFIG. 4 avoids the use of resource-draining fast analogue to digitalconverters by using correlation processors external to the DSP as amethod of simply reducing the required sampling rate of the analogue todigital converters. Only a single correlator (mixer) is shown in each ofthe feedback paths to the in-phase and quadrature channel predistorters(correlators 410 and 412), but the principle may be extended to a numberof correlators and hence a number of orders of distortion as will bediscussed later with reference to FIGS. 5a-5 c. In other respects, theembodiment of FIG. 4 is similar to that of FIG. 1.

It will be apparent to the skilled person that this scheme can beextended to any desired order of distortion generation. It will also benoted that second, fourth and sixth order distortion signals can beextracted at taps 322, 324 and 326, respectively, and these even-orderdistortion signals may be used in forms of predistortion control.

The operation of these correlating processes and their extension tomultiple distortion orders will now be explained with reference to FIGS.5a-5 c. In the embodiment shown in these figures, the I and Q channeldigital inputs are provided to digital signal processor 500 whichcomprises two independent predistorters 510 and 512 which operate on theI and Q channel inputs respectively. The predistorters 510 and 512 areeach constituted as described with reference to FIGS. 2 and 3. Anupconverter 514 frequency upconverts the I and Q predistorted signals toprovide an IF band signal which is then transferred to the analogueportion of the circuit via digital to analogue converter 516. Theanalogue portion of the system functions in the same manner as describedfor the embodiment of FIG. 1 with the exception that the I and Qfeedback paths terminate in splitters 518 and 520 which feed thecorrelation processes (described below).

An advantage of this approach to predistortion generation, is that theprocessing is quite simple, with each of the mixer blocks (310,312, etc)simply being equivalent to a multiplication which involves only a singleinstruction cycle in most DSPs. The predistortion generation mechanismoperates directly on the input signals supplied at 300, and hence nomemory is required to store coefficients, unlike some conventional formsof baseband predistortion generation where the memory requirement may belarge, or if it is reduced, real-time interpolation calculations must beperformed, thereby increasing the processing power requirement.

In the FIG. 1 embodiment, a high speed analogue to digital converter maybe required to sample the IF band feedback signal. The embodiment ofFIG. 4 avoids the use of resource-draining fast analogue to digitalconverters by using correlation processors external to the DSP as amethod of simply reducing the required sampling rate of the analogue todigital converters. Only a single correlator (mixer) is shown in each ofthe feedback paths to the in-phase and quadrature channel predistorters(correlators 410 and 412), but the principle may be extended to a numberof correlators and hence a number of orders of distortion as will bediscussed later with reference to FIG. 5. In other respects, theembodiment of FIG. 4 is similar to that of FIG. 1.

This technique operates by correlating a version of the relevantbaseband distortion component (for example, third order), followingfrequency-offset up-conversion to the IF band and digital to analogueconversion (416,418), with the downconverted (at 414) feedback signalfrom the amplifier output. The control signal acts to minimise thiscorrelation result, as this will minimise the residual distortion at theoutput of the amplifier. The use of a small frequency offset whenupconverting the distortion component ensures that the wanted IF bandcorrelator result is at an appropriate (audio) frequency, thus removingany problems with DC offsets at the correlator output. The audiofrequency result of the correlation may then be sampled by a low samplerate converter (420,422) and detected within the DSP, which eliminatesthe possibility of DC drifts.

It will be appreciated that although this approach uses high samplingrate digital to analogue converters (416,418) to supply the offsetbaseband distortion output, this solution will, however, involve lowercost and lower power consumption than the high sampling rate analogue todigital converters which would otherwise be used in the provision of thefeedback signal to the DSP in FIG. 1 embodiment (even taking intoconsideration the additional cost and power consumption of the lowsample rate analogue to digital converters 420,422). The dynamic range(number of bits) required of the digital to analogue converters 416 and418 will also be much smaller than that required in the analogue todigital converters of the feedback path of the FIG. 1 embodiment. Thisresults in a further cost and power saving.

The operation of these correlating processes and their extension tomultiple distortion orders will now be explained with reference to FIG.5. In the embodiment shown in this figure, the I and Q channel digitalinputs are provided to digital signal processor 500 which comprises twoindependent predistorters 510 and 512 which operate on the I and Qchannel inputs respectively. The predistorters 510 and 512 are eachconstituted as described with reference to FIGS. 2 and 3. An upconverter514 frequency upconverts the I and Q predistorted signals to provide anIF band signal which is then transferred to the analogue portion of thecircuit via digital to analogue converter 516. The analogue portion ofthe system functions in the same manner as described for the embodimentof FIG. 1 with the exception that the I and Q feedback paths terminatein splitters 518 and 520 which feed the correlation processes (describedbelow).

The control processing is carried out independently for the I and Qchannels, thus providing independent quadrature polynomial models of theamplifier characteristic. Since the control scheme for each of thepredistorters 510 and 512 is essentially the same, only that for thepredistorter 512 operating on the quadrature channel digital inputsignal will now be described.

Each of the odd orders of distortion 522,524,526,528 generated inpredistorter 512 are provided to an input of a respective mixer530,532,534,536 to the other input of which is supplied aquadrature-shifted, offset local oscillator signal from generator 538.It will be appreciated that quadrature splitter 540 provides acorresponding in-phase version of the offset local oscillator signal tothe control mechanism for predistorter 510. Returning to the controlmechanism for predistorter 512, the outputs of mixers 530,532,534,536represent the orders of distortion generated within the predistorter 512as upconverted by the offset local oscillator signal. These signals areconverted to analogue signals and are supplied to inputs of mixers542,544,546,548. These mixers correlate the offset-upconverteddistortion orders with the feedback signal provided to splitter 520.

The resulting audio range signals are sampled by a bank of analogue todigital converters and are fed to a further set of correlating mixers550,552,554,556 within the DSP. To the other input of each of thesemixers is supplied a signal derived from the correlation of the outputof offset local oscillator 538 with the signal from the local oscillator558 (in upconverter 514) at 560. The LOs 558 and 538, respectively,could have frequencies of 70 MHz and 70.001 MHz, the output of mixer 560being at 1 KHz, the off-set frequency.

The outputs of correlators 550,552,554,556 are then integrated to supplycontrol signals for the amplitude adjusting elements of the predistorter512 (which were described with reference to FIG. 2). Equally, thecontrol signals produced by the integrators could be used to control thephase adjusting elements of the predistorter 512. In this way, feedbackcontrol of the predistorters 510 and 512 is achieved. Since theintegration and the preceding correlation takes place digitally anypossibility of DC drift or offsets degrading the level of(intermodulation) distortion is eliminated. In order for the processingto remain coherent, it is necessary for the DSP clock and the RF localoscillator(s) to be derived from the same source, or to be phase lockedin some way. The simplest method of achieving this is to drive both theDSP clock and the local oscillator(s) from the same crystal referenceoscillator.

It should be noted that the system could be operated in polar (amplitudeand phase) format in place of the Cartesian (I & Q) format describedabove.

It is possible to include adaptive filtering within the predistorters tocreate a controlled arbitrary amplitude and/or phase versus frequencycharacteristic for each order of distortion generated. This can enablelinearisation of an output signal which would otherwise experienceunequal intermodulation distortion. As shown in FIG. 6, the basicpredistortion scheme, as illustrated in FIG. 2, can be adapted by theinclusion of an adaptive filter 610, etc. in each of the paths forgenerating the various orders of distortion. These filters can beimplemented digitally, and can be of recursive or non-recursive type,and may be adapted using a feedback signal from the output of the RFpower amplifier.

The basic system may also be modified to use a pilot signal which isinjected into the main signal path prior to upconversion andamplification. The pilot tone would be created within the DSP (using,for example, a numerically controlled oscillator) and added prior toupconversion of the I and Q input signals to the IF band. The pilotsignal will be subject to cross-modulation distortion from the inputsignal proper during the upconversion and amplification processes, andthis cross-modulation distortion can be fed back from the output of theRF power amplifier to control the predistorters as described in UKPatent Application 9814391.0. The cross modulation components afflictingthe pilot signal can be minimised using the feedback control mechanismwhich, in turn, leads to minimisation of the related distortion of themain signal due to intermodulation processes.

What is claimed is:
 1. A method of linearizing an output signalcomprising the steps of providing an input signal, digitallypredistorting the input signal using polynomial distortion generationand frequency converting it in succession to provide a predistorted,frequency-converted signal, and amplifying the predistorted,frequency-converted signal to produce an output signal, wherein thepredistorting step comprises producing a predistortion from the inputsignal for introduction to the input signal by generating differentorders of distortion from the input signal and controlling the orders ofdistortion independently.
 2. A method according to claim 1, wherein thepredistortion of the input signal occurs prior to its frequencyconversion.
 3. A method according to claim 1, wherein the frequencyconverting step is a frequency up-converting step.
 4. A method accordingto claim 1, wherein the input signal is in quadrature form comprisingin-phase and quadrature channels, and the predistorting step comprisesthe step of predistorting each channel independently.
 5. A methodaccording to claim 1, wherein the predistorting step comprises the stepof controlling the amplitude and/or phase of the predistortion.
 6. Amethod according to claim 1, wherein the predistorting step comprises astep of controlling the predistortion to introduce a variation ofamplitude and/or phase with frequency into the predistortion.
 7. Amethod according to claim 1, comprising the step of controlling thepredistortion on the basis of a feedback signal derived from the outputsignal.
 8. A method according to claim 7, comprising the step ofintroducing a pilot signal into the input signal and wherein the step ofcontrolling the predistortion on the basis of a feedback signalcomprises the step of monitoring distortion of the pilot signal in theoutput signal.
 9. A method according to claim 7, further comprising thestep of using the predistortion together with the feedback signal togenerate control signals for the predistortion step.
 10. A methodaccording to claim 9 wherein the step of using the predistortiontogether with the feedback signal to generate control signals comprisesthe step of correlating the predistortion with the feedback signal. 11.A method according to claim 10, comprising the step of frequencyconverting the predistortion prior to the correlating step.
 12. A methodaccording to claim 11, wherein the step of frequency converting thepredistortion comprises the step of mixing the predistortion with afirst signal at a first frequency.
 13. A method according to claim 12,wherein the step of frequency converting the input signal comprises thestep of mixing it with a signal at a second frequency and thecorrelating step comprises the step of mixing the frequency convertedpredistortion with the feedback to produce intermediate signals.
 14. Amethod according to claim 13, wherein the correlating step comprises thestep of correlating the intermediate signals with a signal whosefrequency is the difference of the first and second frequencies.
 15. Amethod according to claim 9, wherein the step of using the predistortiontogether with the feedback signal to generate control signals isperformed at least partly in an analogue signal domain.
 16. A methodaccording to claim 1, wherein the predistorting step comprises, mixingor multiplying the input signal with itself to generate a distortionsignal.
 17. A method according to claim 1, wherein the predistortingstep comprises generating different orders of distortion by mixing theinput signal with itself repeatedly.
 18. A method according to claim 1,wherein the predistortion occurs in a digital signal processor.
 19. Amethod according to claim 1, wherein the frequency conversion of theinput signal occurs in a digital domain.
 20. A method of transmittinginformation wirelessly, comprising the step of manipulating theinformation to produce an input signal and processing the input signalby the method of any preceding claim in order to generate an outputsignal for transmission from antenna means.
 21. Apparatus forlinearising an output signal comprising predistorting means fordigitally predistorting an input signal using polynomial predistortiongeneration and frequency converting means operating in succession on aninput signal to provide a predistorted, frequency-converted signal, andamplifying means for amplifying the predistorted, frequency-convertedsignal to produce an output signal, wherein the predistorting meansproduces a predistortion from the input signal for introduction to theinput signal by generating different orders of distortion from the inputsignal and controlling the orders of distortion independently. 22.Apparatus according to claim 21, wherein the predistorting meansoperates on the input signal prior to the frequency converting means.23. Apparatus according to claim 21, wherein the frequency convertingmeans comprises frequency up-converting means.
 24. Apparatus accordingto claim 21, wherein the input signal is in quadrature form comprisingin-phase and quadrature channels, and the predistorting means comprisesmeans for predistorting each channel independently.
 25. Apparatusaccording to claim 21, wherein the predistorting means comprises meansfor controlling the amplitude and/or phase of the predistortion. 26.Apparatus according to claim 21, wherein the predistorting meanscomprises means for controlling the predistortion to introduce avariation of amplitude and/or phase with frequency into thepredistortion.
 27. Apparatus according to claim 21, comprising controlmeans for controlling the predistortion on the basis of a feedbacksignal derived from the output signal.
 28. Apparatus according to claim27, comprising injecting means for introducing a pilot signal into theinput signal and wherein the control means comprises means formonitoring distortion of the pilot signal in the output signal. 29.Apparatus according to claim 27, further comprising control signalgenerating means for using the predistortion together with the feedbacksignal to generate control signals for the predistorting means. 30.Apparatus according to claim 29, wherein the control signal generatingmeans comprises correlating means for correlating the predistortion withthe feedback signal.
 31. Apparatus according to claim 30, comprisingpredistortion frequency converting means for frequency converting thepredistortion prior to its correlation by the correlating means. 32.Apparatus according to claim 31, wherein predistortion frequencyconverting means comprises predistortion mixing means for mixing thepredistortion with a first signal at a first frequency.
 33. Apparatusaccording to claim 32, wherein the frequency converting means forfrequency converting the input signal comprises means for mixing it witha signal at a second frequency and the correlating means comprises meansfor mixing the frequency converted predistortion with the feedback toproduce intermediate signals.
 34. Apparatus according to claim 33,wherein the correlating means comprises means for correlating theintermediate signals with a signal whose frequency is the difference ofthe first and second frequencies.
 35. Apparatus according to claim 29,wherein the control signal generating means operates at least partly inan analogue signal domain.
 36. Apparatus according to claim 21, whereinthe predistorting means comprises means for mixing or multiplying theinput signal with itself to generate a distortion signal.
 37. Apparatusaccording to claim 21, wherein the predistorting means comprises ordergenerating means for generating different orders of distortion by mixingthe input signal with itself repeatedly.
 38. Apparatus according toclaim 21, wherein the predistorting means is implemented by a digitalsignal processor.
 39. Apparatus according to claim 21, wherein the meansfor frequency converting the input signal operates in a digital domain.40. Apparatus for transmitting information wirelessly, comprising meansfor manipulating the information to produce an input signal andapparatus of claim 21 for processing the input signal in order togenerate an output signal for transmission from antenna means.